PPro 2010 leaves a file with the extension "icache" -What does this file do? Is. it worth  keeping ?

What is the full file name? What folder is it in? More clues, please! :O)

Every scene file I render. the file is put in the directory where I keep scene files.

are you using IDL ? - looks like it might be the irradience map data judging by the name

At a guess those files are caching the Irradience data, and as such are temporary files.

Yes. I use IDL. I surmised they might be associated with Irradiance Cache. Will  keeping them speed rendering?

depends on whether or not Poser reuses the irradiance data or not.  I don't have it so I couldn't say for sure but no doubt someone will have an answer

Speaking only theoretically, I don't think it re-uses the cache. If it did, there would probably be an option to clear it, and I wouldn't see red-dot IDL pre-calc a second time with no scene changes.

"*When working on a static scene, you can also save an irradiance map and then reuse the map
over and over to render it from different point of view.

You can also use this feature for some more interesting effects. For instance, you could render
the irradiance map for a scene without texture so that the Global Illumination solution is faster
and less noisy and then use that* irradiance *map to render your scene with the texture applied."

**

So, the files are probably associated with irradiance caching, and they are probably a waste of space.

Does anyone have any idea how they might be used in subsequent renders of the same scene but with characters whose positions and poses have changed, in order to reduce the time used for pre-calculation during rendering?

I should mention that these are big files, at least as big as the scene file itself.

The person to ask would probably be Stewer, so you might try sending him an IM.